The invention relates to a device for inspecting an interference filter for a projection television display tube.
As compared with a television having a conventional direct-view screen, currently known projection television systems have a moderate brightness, a colour rendition which is not very good, a moderate contrast and some loss of resolution. The moderate colour rendition is mainly caused by the fact that the conventionally used Tb-activated green phosphor has too high a contribution of orange and red spectral lines. The defocus is caused to a considerable extent by the chromatic aberration in the lenses, which is particularly noticeable for phosphors having a wide range of spectral emissions, such as the conventionally used green (Tb-activated) and blue (ZnS:Ag) phosphors.
To cope with these problems it has been proposed, for example in EP-A No. 0,174,320 to which U.S. Pat. No. 4,634,926 corresponds, to arrange an interference filter between the phosphor layer and the glass of the display screen. Within the range of visible light, the proposed interference filter is a short wavelength transmission filter. It transmits light of the desired wavelength in the forward direction and at angles of up to approximately 35.degree. to 40.degree. to the normal. At a larger angle to the normal of the display screen the interference filter reflects the light into the phosphor layer. It is scattered there and may subsequently leave the phosphor layer approximately in the forward direction so that it is passed by the filter. Thus, the interference filter causes an increase of the light of the desired wavelength in the forward direction.
At a smaller wavelength the angular range with respect to the normal at which light still passes is larger, so that the relative increase of the light in the forward direction (within the angle of acceptance of the projection lens) is smaller. At a longer wavelength the angular range is smaller or the light is even completely blocked. Thus, the interference filter operates colour-selectively in such a way that the colour of the light transmitted by the tube improves, the brightness increases and the chromatic aberration in the lenses decreases.
It will be evident that the quality of the image, and particularly the colour rendition, depends on the quality of the interference filters. It is therefore necessary to inspect at least a representative part of the filters from each production batch.
A filter can be measured by determining its transmission and reflection properties at a number of wavelengths. This method may be performed without any problem in a laboratory, but it is too complicated, time-consuming and hence too costly for use in mass-manufacturing in a factory. It is thus desirable to have a simple, fast and inexpensive test in which a minimum number of parameters to be measured suffices.